BIOCHEMICAL BASIS OF RESISTANCE OF TOBACCO CALLUS-TISSUE CULTURES TO HYDROXYPHENYLETHYLAMINES

It has been reported that hydroxyphenylethylamines, such as tyramine a nd octopamine, are toxic to tobacco (Nicotiana tabacum L.) callus cult ures grown in the presence of auxins, whereas calli grown in the prese nce of cytokinins and crown gall cultures are resistant to these amine s (P. Christou and K.A. Barton [1989] Plant Physiol 89: 564-568). In a n attempt to understand the underlying mechanism of this resistance, w e compared the fates of tyramine in tyramine-sensitive and tyramine-re sistant tobacco tissue cultures (cv Xanthi nc). The very rapid formati on of black-colored oxidation products from tyramine in sensitive tiss ues suggested that the toxicity might be caused by the oxidation of ty ramine by phenol oxidases present in the tissues or released into the medium after subculture. This was confirmed through many indirect proc edures (effect of exogenously added tyrosinase, induction of polypheno l oxidase [PPO] activity by auxin, etc.). The study of tyramine struct ure-activity relationships further suggested that the toxicity of tyra mine might be due to the formation of indolequinones after oxidation b y PPO. Subculture of calli grown on 2,4-dichlorophenoxyacetic acid in a medium containing benzyladenine triggered a slow decrease in PPO act ivity and dramatic increases in peroxidase and tyramine hydroxycinnamo yl transferase (THT) activities. THT was undetectable in calli grown o n 2,4-dichlorophenoxyacetic acid but very active in tyramine-resistant crown gall cultures. Moreover, when [H-3]tyramine was fed in vivo to tyramine-resistant tissues, it was rapidly integrated into cell walls in the wound periderm formed at the periphery of the calli. Both the c onjugation of tyramine and its integration into cell walls could compe te with the formation of toxic quinones and therefore play a part in t he resistance. Thus, it seems likely that the control of the toxicity of hydroxyphenylethylamines by cytokinins results primarily from chang es in the metabolism and the compartmentation of these amines.